If you remember my description of the observing plan, I figured out the times when Pluto would present the same hemisphere as seen during the previous set of observations. I also computed how long the window of opportunity was for each time. Then, I prioritized them according to which hemisphere I thought would be most interesting. This is a sound plan and would have worked just fine but for one minor flaw. The window of opportunity I gave was about 2 hours long. In 2 hours, Pluto rotates about 5 degrees of longitude. I figured this was long enough a window to permit scheduling the observation while minimizing the amount of rotation possible between the new and old pictures. It turns out that this duration is just a little too short. You may remember, an orbit of HST takes about 94 minutes during which Pluto can be seen during about 45 minutes. Here's a crude graph that should illustrate the problem:
|-------- orbit ------------|-----------orbit-----------| oooooooooooooo | oooooooooooooo <- Pluto visible ---------------------------------- <- my window
Remember that the observation must fit in one orbit of HST. Each orbit has a time where the object can be seen (o's above). That must then fit within my window. An orbit lasts 94 minutes and a viewing window lasts 47 minutes. That puts a minimum length of 141 minutes on a window to make sure it will fit. My 120 minute window in this case starts too late for the first orbit and finishes too early for the second orbit in the worst case. You can see that if the window slides to the right or left then there can be instances where this will work, but it won't always work.
This sort of oversight is easy to make and normally I would get feedback from STScI that I goofed. At this point, the processing of this observation began to depart from normal operating procedures. Instead of getting a message that I goofed, window #2 on my list (March 4) was chosen for HST even though it didn't fit. Why #2? Well, it took quite a few e-mail messages back and forth with Tony Roman and Alex Storrs to get to the bottom of the problem. I thought that #2 was picked because #1 didn't work. As it turns out, #1 didn't work but neither did #2. They should have continued down the list until they found one that would work.
What happened? Well, this observation is being planned specially for the Live live broadcast to experience our first look at the data. Can you imagine what would happen if for some reason the observation failed and there was no new picture of Pluto? There are lots of ways that an observation can fail and it does happen more often than you would think. The scheduling committee at STScI decided (without consulting me) that they would put the observation as early as possible so that there would be time to repeat the observation if the first try failed. The first possible day was March 4 and I happened to have a window on that day so it was chosen.
After all this had already happened and the observation was scheduled, I was contacted to see if the mismatch between my window and the Pluto observing window would harm the observation. Unfortunately, this notification happened too late to make any changes to the schedule. Anyway, I've since figured that the slight mismatch probably won't hurt our observations and I really should have made my 2 hour window just a little bit longer. In the end, we should get what we wanted but it happened in a strange way.
These experiences I've just outlined detail all of the discussions I had with STScI about the planning and scheduling of the Pluto picture. You might be wondering what else has been going on. After all, it's taken me far too long to get around to writing this stuff down. That's the subject for my next journal entry.